1. Field of the Invention
This invention relates to improvements in an electroluminescence (EL) element which emits light when an AC or DC voltage is applied thereto, and more particularly it relates to improvements in a thin film EL element having a light emitting layer formed by thin film formation method such as electron beam deposition or sputtering.
2. Description of the Prior Art
FIG. 2 is a sectional view of a prior art thin film EL element adapted to be driven by AC. This thin film EL element 1 has a light-premeable substrate 2 made of transparent glass, and a transparent electrode 3 made, e.g., of In.sub.2 O.sub.3 --SnO.sub.2. Formed on this transparent electrode 3 is an insulation layer 4, on which is formed a light emitting layer 5 made, e.g., of ZnS:TbF.sub.3, the light emitting layer 5 being sandwiched between an insulation layer 6 formed on the upper surface thereof and the insulation layer 4. The first insulation layer 4, the light emitting layer 5 and the second insulation layer 6 are formed by sputtering. The first and second layers 4 and 6 are formed of a non-crystalline or polycrystalline dielectric film of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 or the like. As the other electrode, an electrode 7 is formed in a lattice pattern on the second insulation layer 6 so as oppose to the transparent electrode layer 3.
Since EL elements function as display devices, first, high brightness and high efficiency are required and, second, it is preferable that they can be driven with low voltage. The efficiency and drive voltage of such thin film EL elements depend largely on the crystallinity of ZnS which forms the light emitting layer, and it is desirable that the crystallinity of the light emitting layer be superior. In the conventional thin film EL element 1 shown in FIG. 2, the light emitting layer 5 is formed by sputtering on the surface of the first insulation layer 4 made of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3. However, normally where an insulation layer is formed of a dielectric film of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3, and where the light emitting layer 5 is formed thereon, its crystallinity in early periods of growth is poor, resulting in the portion A of the light emitting layer 5 becoming a so-called dead layer, as shown in FIG. 2, which presents an obstacle to increasing brightness and efficiency and to reduction of drive voltage.
On the other hand, in a DC type thin film EL element driven with DC, not shown, there is no need of insulating the light emitting layer from the electrodes; thus, its arrangement is the same as that of the AC-driven thin film EL element 1 shown in FIG. 2 excluding the first and second insulation layers 4 and 6. In this DC-driven thin film EL element, like the AC-driven thin film EL element, it is necessary to form a light emitting layer having satisfactory crystallinity. As an example thereof, it has been reported that a thin film EL element of high efficiency can be obtained by forming a vapor-deposited film of ZnSe as a buffer layer on a transparent electrode and then forming thereon a light emitting layer of ZnS:TbF.sub.3. FIG. 3 is a graph showing the relation between the thickness and light emitting efficiency of a light emitting layer of ZnS-TbF.sub.3, the dot-and-dash line indicating an example of a thin film EL element having a transparent electrode of In.sub.2 O.sub.3 --SnO.sub.2, a ZnSe vapor-deposited film, a ZnS:TbF.sub.3 light emitting layer and an Al electrode, which are successively formed in the order mentioned. The dotted line indicates a comparative example prepared by removing the ZnSe vapor-deposited film from the thin film EL element shown by the solid line. As is clear from FIG. 3, the element with the buffer layer of ZnSe attains satisfactory light emitting efficiency from the very thin region where the thickness of the Zns:TbF.sub.3 which is the light emitting layer is about 0.2 .mu.m. This indicates that the ZnSe vapor-deposited film has satisfactory crystalline structure and forms a substrate for the light emitting layer, enabling the formation of a thin film EL element of high efficiency to be attained. Thus, the ZnSe vapor-deposited film may be taken as being capable of improving the crystallinity in early periods of growth of the light emitting layer of ZnS. Therefore, such concept would be applicable also to the AC-driven thin film EL element.
However, in an electrical circuit, the ZnSe vapor-deposited film serving as a buffer layer would be connected in series with the light emitting layer of ZnS, and application of voltage would result in a voltage drop due to the ZnSe vapor-deposited film. This voltage drop must be minimized in order to reduce the required drive voltage. From the standpoint of reduction of drive voltage, it goes without saying that it is better not to form any ZnSe vapor-deposited film, even if such resistive ZnSe vapor-deposited film caused a minimal of voltage drop. The same may be said not only of the DC-driven thin film EL element but also of the AC-driven thin film EL element.